In recent years, there is an ever-increasing demand for a reduction in the weight of automobile bodies, in addition to the comfort and safety of the automobiles, which requires an increase in the strength of thin steel sheets utilized in automobile structures. Further, in the production of components of the automobile body, simplification, and continuous operation, of the production process by a reduction in the number of forming steps, and by one-body pressing, are considered technical requirements. When the thin steel sheet, among steel products used in such forming, is particularly taken into consideration, the selection criterion of the steel product is that the steel product has good formability. Stretchability, deep drawability, stretch-flange ability, and bendability are also required of the thin steel sheet. In this connection, good deep drawability, in addition to stretchability, is required in order to make it possible to prepare components having a complicated shape, such as interiors of automobiles, requiring only a small number of steps or one-body pressing.
The material properties governing the stretchability are elongation and work hardening coefficient (n value). In recent years, a high-strength multiple phase steel sheet comprising a mixed microstructure of ferrite, bainite, and austenite has been proposed as a steel sheet excellent in the above properties. This steel sheet utilizes "transformation induced plasticity" which is a phenomenon such that austenite remaining at room temperature is transformed to martensite at the time of forming, resulting in high ductility. Japanese Unexamined Patent Publication (Kokai) No. 61-157625 discloses, as a process for producing a high-strength steel sheet, a process for producing a thin steel sheet, such as a steel sheet for automobiles which should be inexpensive and mass-produced. In this prior art, Si is added to inhibit the precipitation of carbides, and ferrite transformation (bainite transformation) at low temperature is allowed to proceed to effectively enrich C in untransformed austenite, thereby stabilizing the austenite. Further, there is a report that the volume fraction and stability of retained austenite are important for providing high ductility in this steel (TETU TO HAGANE, 78 (1992) p.1480). However, no mention is made of deep drawability.
On the other hand, the Lankford value (r value) determined by a uniaxial tensile test, rather than elongation and the n value, is generally used as a material property governing the deep drawability. In general, the deep drawability of a material is tested in terms of deep drawing to a cylindrical cup. It is valuated using a formable range of blank holder force between the minimum force which can restrain wrinkles in the flange portion and the maximum force which can prevent rapture at the shoulder portion of the punch. A material having excellent deep drawability has high breaking proof stress in the shoulder portion of the punch and low shrink flanging deformation resistance in the flange portion. According to the theory of plasticity, a material having a high r value is characterized by having high fracture strength in a deformed state around plain strain in the shoulder portion of the punch and low deformation resistance under shrink flanging deformation in the flange portion. The r value is governed by a texture of the sheet, and, hence, in the development of the conventional deep drawable steel sheet, attention has been drawn mainly to the regulation of the texture. In recent years, however, that a steel utilizing deformation induced transformation of retained austenite has excellent drawability has been reported (SOSEI TO KAKO, 35-404 (1994) p.1109). This suggests that a variation in stability of the retained austenite depending upon the type of deformation is important for the deep drawability of this type of steel.
For a high-strength steel sheet having a tensile strength exceeding 440 MPa, it is difficult to attain a combination of strength with regulation of the texture at a production cost comparable to that of the prior art, and, consequently, no steel sheet having satisfactory deep drawability has been developed in the art. Therefore, the application of a high-strength steel sheet having a tensile strength of not less than 440 MPa to components produced mainly by deep drawing, such as components for inner panels of automobiles, is very difficult. Also in the above Japanese Unexamined Patent Publication (Kokai) No. 61-157625 as prior art, the high-strength steel sheet produced has high ductility and n value, and, hence, among various types of formability, the stretchability is particularly excellent. However, the deep drawability is not studied at all, and the high-strength steel sheet is unsatisfactory for the application thereof to components having a complicated shape requiring deep drawability, such as inner panels of automobiles. Further, in this steel sheet, some types of press forming cause age cracking, of articles prepared by press forming, called "season cracking" or "longitudinal cracking," posing a problem when this steel sheet is applied to press forming involving drawing.
Further, in the deep drawing of a high-strength steel sheet, the load necessary for forming is increased, which causes problems such as lack of loading capacity of a pressing machine and galling caused by sliding under high face pressure. For this reason, materials which, despite high strength, can be formed into articles under low load has been desired in the art.
In "TETSU TO HAGANE, 78 (1992) p.1480" cited above, deep drawability is not studied at all. "SOSEI TO KAKO, 35-404 (1994) p.1109" reports the influence of stability of retained austenite on the deep drawability for steels having tensile strength on the order of 600 MPa. It, however, does not clarify the influence of the volume fraction and hardness of each phase on the deep drawability. Further, the technical problems, such as season cracking, loading capacity of the pressing machine, and galling, remain unsolved.
The present invention has been made with a view to eliminating the above problems, and an object of the present invention is to provide a steel sheet, suitable for deep drawing, which, unlike the conventional high-strength steel sheet, can be deep-drawn at a lower forming load while avoiding the occurrence of galling and season cracking.
The term "steel sheet" as used herein is intended to mean a steel sheet which, in order to improve the conversion treatability, corrosion resistance, and press formability, has been subjected to various treatments such as plating with Ni, Zn, or Cr as a main component, formation of a film of an organic compound or an inorganic compound, or coating of a lubricant.